Snowpack below average in southwest Montana

BOZEMAN – As of April 1, the snowpack in most Montana river basins was slightly below normal, but a weeklong weather system starting April 8 improved the average statewide, according to snow survey data from the USDA Natural Resources Conservation Service.

The NRCS shifted its normals period this year, and is now excluding the wetter-than-average decade of 1971-1980 and including the drier 2001-2010 decade.

“That is going to make everything [this year] look wetter than it would have in previous years with the old averages,” said Brian Domonkos, NRCS water supply specialist. “So, 96 percent this year would have been 88 percent last year, with the old normals… The percents of average are markedly changed.”

That, he explained, is why the mountains may look much drier than the snowpack numbers this year would indicate.

The snowpack in the mid- and upper elevations of the basins in southwest Montana is in fact below average, Domonkos said, even compared to last year, which was also a low year. These elevations yield the bulk of the state’s water supply, so continued monitoring will determine the timing and volume of the upcoming runoff.

This was caused in part by nearly a month of high pressure in central and southern Montana, which led to above average temperatures and below average precipitation.

However, the Gallatin and Yellowstone river basins both saw significant increases during the early April storm, Domonkos said, with the Gallatin up 6 percent over the entire basin, to 96 percent of average as of April 15.

The upper Gallatin River Basin – which is everything above Gallatin Gateway – is at 100 percent of average, and the Hyalite watershed, which drains into the Gallatin in Belgrade, is now at 98 percent, Domonkos said.

The storm had a significant effect on the Yellowstone above Billings, which is up 5 percent to 89 percent. Above Livingston the Yellowstone is at 92 percent. The Madison and Jefferson rivers, which didn’t see as much precipitation that week, are at 91 and 85 percent of average, respectively.

Given the current snowpack and near normal expected precipitation and temperatures in the coming weeks, the NRCS is predicting stream flows will be below average this year. However, snowpack typically reaches its maximum in April, and weather this month will impact their timing and volume.